Throughout aviation history the accumulation of ice on the critical surfaces of aircraft wings has presented an issue. Indeed, a small amount of ice accumulation on the leading edge of an aircraft wing can impact the lift and drag characteristics of a surface. Accordingly, the detection of aircraft icing is common procedure, which is generally required to comply with Federal Aviation Administration (FAA) regulations, such as those within the supercooled liquid water envelopes defined by 14 C.F.R. Part 25, Appendix C and Appendix O.
Visual identification of icing conditions by the flight crew is sometimes possible, but is typically quite difficult to do while flying at night, when it is dark outside the aircraft, or when visibility is otherwise poor. Although various systems and technologies exist for ice detection on aircraft, the limitations of such systems and technologies often allow ice to form on the leading edge of a wing in the temperature region just below freezing. Because such ice is able to form prior to detection by certain types of ice detection sensors, this temperature region is sometimes referred to as a zone of non-detection.
Some aircraft, such as the Boeing 737, do not typically have an ice detection system but rely solely on the flight crew and visual means for detection of icing conditions. FAA regulation 14 C.F.R. § 25.1403 specifies a means of determining the presence of ice accretion on aircraft wings during night operations. A typical means for meeting this requirement uses flush-mounted lights installed on the fuselage which are pointed in the direction of the wing leading edge. These lights may use an incandescent or light-emitting diode (LED), visible light source and are intended to project enough light onto the leading edge of the wing to be a visual aid for the flight crew to see the wings. Although these lights improve the ability to see the wings, it is nonetheless often difficult or impractical for the flight crew to detect ice in a timely manner for the activation of aircraft ice protection systems, particularly since ice and the wing surface are often difficult to visually differentiate, e.g., from a distance and/or when the wing is wet.